This invention relates to a refrigeration method and apparatus and is particularly concerned with the liquefaction of a permanent gas, for example nitrogen or methane.
A permanent gas has the property of not being able to be liquefied solely by increasing the pressure of the gas. It is necessary to cool the gas (at pressure) so as to reach a temperature at which the gas can exist in equilibrium with its liquid state.
Conventional processes for liquefying a permanent gas or for cooling it to or below the critical point typically require the gas to be compressed (unless it is already available at a suitably elevated pressure, generally a pressure above 30 atmospheres) and heat exchanged in one or more heat exchangers against at least one relatively low pressure stream of working fluid. At least some of the working fluid is provided at a temperature below the critical temperature of the permanent gas. At least part of each stream of working fluid is typically formed by compressing the working fluid, cooling it in the aforesaid heat exchanger or heat exchangers, and then expanding it with the performance of external work ("work expansion"). The working fluid may itself be taken from the high pressure stream of permanent gas, or the permanent gas may be kept separate from the working fluid, which may nonetheless have the same composition as the permanent gas.
Typically, the liquefied permanent gas is stored or used at a pressure substantially lower than that at which it is taken for isobaric cooling to below its critical temperature. Accordingly, after completing such isobaric cooling, the permanent gas at below its critical temperature is passed through an expansion or throttling valve whereby the pressure to which it is subjected is substantially reduced, and a substantial volume of so called "flash gas" is produced. The expansion is substantially isenthalpic and results in a reduction in the temperature of the liquid being effected. Generally, one or two such expansions are performed to produce flash gas and liquefied permanent gas in equilibrium with its vapour at a storage pressure. Generally, the thermodynamic efficiency of commercial processes for liquefying permanent gas is relatively low and there is ample scope for improving such efficiency. Considerable emphasis in the art has been placed on improving the total efficiency of the process by improving the efficiency of heat exchange in the process. Thus, prior proposals in the art have centred around minimising the temperature difference between the permanent gas stream and the working fluid stream or streams being heat exchanged therewith.
We have now found a way of increasing the efficiency of the isenthalpic expansion stage of the liquefaction process. This increase of efficiency is not merely of intrinsic value: it also enables more favourable conditions to be set for the work expansion (or at least the lower or lowest temperature work expansion) of working fluid and therefore makes it possible to achieve an improvement in the overall thermodynamic efficiency of the liquefaction greater than that achievable for the isenthalpic expansion alone.